중앙데일리

TechnoMart shaken by gym: Test

“12층서 태보했더니 38층 상하진동”

July 20,2011

A group of 23 middle-aged people did vigorous exercises at the 39-storey TechnoMart complex, and managed to make it shake.

They were part of a demonstration to prove the owner’s claim that ten minutes of intense shaking on July 5, which led to the evacuation of the complex near Gyangbyeon Station, in eastern Seoul, for two days, was caused by an exercise session in a fitness center on the 12th floor.

The demonstration was held at 3:30 p.m. yesterday, and people working on the complex’s upper floors said they were all shook up.

“I felt almost the same vibrations today as on July 5,” a worker on the 31st floor surnamed Lee said in an interview with the Korea JoongAng Daily.

Two Korea JoongAng Daily reporters also felt tremors on the 31st floor. The minor vertical vibration felt like riding a boat over light waves. If it had gone on for several minutes, it might have caused discomfort, dizziness or nausea.

On July 5, the upper 19 floors of the complex, which has apartments and a popular shopping mall, shook for 10 minutes. After having the complex inspected, owner Prime Group said the cause was 17 middle-aged people in a 12th floor fitness center performing Tae Bo exercises, a mixture of boxing and taekwondo that became popular in the 1990s.

“We tentatively concluded that the group exercise in the fitness center on the 12th floor of the building caused the shake,” Park Heung-soo, CEO of Prime Group said at the briefing yesterday. “After conducting more inspections to find other possible reasons, such as wind or machinery motion, we will make a final report on the cause of the shaking three months later.”

At yesterday’s demonstration, 23 middle-aged people were recruited to do the same Tae Bo exercise as done on July 5. They exercised to the same song, “The Power” by Snap.

The demonstration was done in three parts: two minutes of warm-up exercises, three minutes of light Tae bo and three minutes of vigorous Tae bo exercises.

Lee Dong-geun, professor at Sungkyunkwan University’s Architectural Engineering Department said at the briefing: “On July 5, the teacher was brand new. He started his or her job on that day and was teaching 17 people. Witnesses say the teacher was doing very hard motions, such as pounding the floor with feet, and the duration of the class was longer than usual, about 20 minutes.”

The teacher has disappeared because he doesn’t want media attention. The original 17 gym rats also refused to participate in the recreation of the July 5 episode.

“When the natural frequency of vibration of a building matches the frequency of people’s group activities, such as group stepping or jumping, it gives a big shock to the building and could even shake it,” Lee explained. “We call the phenomenon [mechanical] resonance.”

“The bigger the shock, the more it shakes the upper part of the building than the lower part,” Lee said. “That’s the reason why people working on lower floors felt very little vibration.”

In 1831, the Broughton Suspension Bridge in England collapsed due to mechanical resonance induced by troops marching over the bridge in unison. In June 2000, the Millennium Bridge in London shook when a massive group of people flocked to the newly-built bridge.

Hwang Hyeon-sun, a director of Prime Group who works on TechnoMart’s 38th floor, said the building usually swayed a little in heavy winds or storms.

“On July 5, the building shook more than usual,” Hwang said. “The shaking from the practice demonstration was similar to the shaking on the 5th.”

Experts also investigated the 4-D theatre on TechnoMart’s 10th floor, but said it wasn’t involved in the shaking.

Concerns remain, however, among workers in the building. About 50,000 people visit TechnoMart every day.

“People barely believe the building is safe, so I hope reputable companies hold press conferences for the sake of the 3,000 merchants,” said an employee of a restaurant on the 9th floor.